In known wood chip drying systems, wood dust is usually burnt in the furnace. The hot flue gases occurring are routed into a mixing chamber and are to be mixed there with vapor which is also called back vapor. Owing to mixing, the temperature of the drier gas occurring falls to approximately 380° C. to 420° C. The drier gas is subsequently supplied to the drier where the wood chip is dried.
The vapor occurring is delivered to a cyclone and then partially recirculated into the mixing chamber. The dried wood chip is then blended with glue and pressed into a wood material board. The disadvantage of wood chip drying systems of this type is that the wood material boards produced from the wood chip may give off volatile organic substances.
Also known hot-gas generation and drying of the wood chip take place in that primarily wood dust with a dedicated combustion air supply is burnt in a combustion chamber. The approximately 900-degree flue gases from this combustion enter a mixing chamber in which the back vapor, as it is known, the secondary air for cooling and, if appropriate, external hot gas are introduced. In the mixing chamber, the drying hot gases to be introduced into the chip drier are set at the necessary requirements (temperature of approximately 350-480° C., overall volume flow and moisture content). The hot gas volume flow or drying air volume flow thus prepared is sucked via the drier drum by means of a suction draft fan. In the drier drum, the wood chip is dried in direct contact with the hot gases. The suction draft fan conveys the overall hot gas volume flow involved in the drying process to a filter system which is designed predominantly as a cyclone separator, but also as an electrostatic wet filter. This filter system primarily separates only solid particles to a limited extent. Upsteam or downsteam of this filter system, a substream, called the back vapor volume flow, is delivered to the abovementioned mixing chamber again.
The drying of the wood chip requires a balanced temperature, energy and volume flow equilibrium which must be set as a function of the product (chip size, throughput, moisture content, type of drier).
The dried wood chip is separated downstream of the drier drum, blended with glue and pressed into a wood material board. The disadvantage of wood chip drying systems of this type is that the wood material boards produced from the wood chip may give off volatile organic substances arising from resins and terpenes.
The hot gases for drying the wood chip partially absorb the resins and terpenes contained in them from the wood. The best effect for this purpose is achieved in that the drier hot gases are free of ballast substances of the same type at the entrance of the drier. However, the hot gases at the drier entrance are mixed according to known methods with back vapors which have already participated in the drying process. The absorption of resins and terpenes by the mixing volume flow is therefore reduced.
It is also known to route the vapors completely in a closed circuit, that is to say the drier hot gases conducted into the drier are generated entirely from vapors, mixed with the exhaust gases from the combustion of primary fuels. To improve the method in energy terms, the vapors are partially conducted into the combustion chamber and treated thermally here at approximately 850° C. The other vapor fraction is conducted via a tube bundle heat exchanger and is heated here to approximately 380 to 450° C., while at the same time the (50 degree) hot gases from the combustion chamber are cooled. The vapors heated via the heat exchanger are not treated thermally on account of the temperature level. Major disadvantages of this type of hot gas generation by means of the tube bundle heat exchanger systems are low availability on account of contamination of the heat exchanger surfaces. Only part of the vapors pass through thermal treatment. The combustion air for burning the primary fuel is sucked in externally. The thermal efficiency of the method is relatively unfavorable.
It is therefore also known to route the vapors entirely in a closed circuit, that is to say the drier gas conducted in the drier was generated entirely from vapor in which volatile compounds were oxidized thermally. The disadvantage of this is that a wood chip drying system of this type has low availability.
U.S. Pat. No. 5,983,521 discloses a wood chip drying system in which the back vapors are recirculated completely into the combustion chamber. To save energy, the back vapors are conducted through a recuperator which is fed with hot gas from the furnace. All the back vapors are thereby treated thermally in the combustion chamber. The disadvantage of this is that the efficiency of the furnace falls, since all the back vapors have to be routed completely through the furnace.
A garbage drying system is known from DE 197 28 545 A1. Contrary to wood drying systems, the moisture content and the terpene and fine dust content do not play a relevant part in garbage drying systems, and therefore the apparatus described there is not suitable for drying wood chip. CH 133 536 discloses a method for drying bulk goods which is of the multistage type. The publication does not describe a regeneratively or catalytically active heat exchanger.
DE 29 26 663 discloses a method in which the vapors are cooled in order to achieve condensation of the water and of the entrained terpenes. The disadvantage of this is the high outlay for cooling down the vapors.
WO 01/59381 discloses a wood chip drying system in which part of the vapors is recirculated into the combustion chamber. The disadvantage here is that part of the terpenes contained in the back vapors may settle on the product to be dried, so that the wood chip, after leaving the drier, has an increased content of volatile organic constituents.